The Microstructure, Thermal Conductivity and Joining Strength of Eutectic Bonded Cu/Al2O3 Laminates

The present study applies the eutectic bonding, so called direct bonding, to join copper to alumina. The oxygen plays an important role on the joining of the metal to ceramic, and the ceramic/metal interfaces with various oxygen concentrations are investigated. In order to introduce various oxygen concentrations into copper, several pre-oxidation treatments were adopted. The oxidation behavior of Cu is also investigated in this work. Cu2O and CuO form at the copper surface during oxidation. The oxidation is controlled by diffusion. The eutectic bonding of copper to alumina is achieved at 1075℃ in nitrogen atmosphere. Eutectic melt forms and wets both copper and alumina, therefore intimate bonding is obtained after cooling. HRTEM is used to observe the microstructure at the interface. CuAlO2 is found at the Cu/Al2O3 and Cu2O/Al2O3 interfaces. Two chemical reactions for the formation of CuAlO2 are proposed. Thermodynamics and kinetics are used to elucidate the formation of CuAlO2 at the interface. Since the processing window for the stable CuAlO2 is narrow, to maintain a sufficient oxygen concentration at the Cu/Al2O3 interface is critical. The interfacial phases, Cu2O and CuAlO2, have contradictory influences on the joint properties. The presence of CuAlO2 improves the interfacial strength by crack pinning mechanism whereas Cu2O reduces the interfacial strength and introduces large residual stress. An adequate oxygen content should be used to join Cu to Al2O3 for optimum properties. Flash method and flexure bending test were used to characterize the laminate properties. The bilayer Cu/Al2O3 laminates fabricated in the present study exhibit a thermal conductivity above 30 Wm-1K-1 and a steady-state energy release rate of 23 Jm-2. Since the thermal conductivity and interfacial strength of the Cu-Al2O3 laminates are high, the use of the laminate as thermal dissipation substrate is high.